Tool - Plumb Bob
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Typical plumb bobs are 8 oz or 16 oz. The best choice is 8 oz because the 16 oz ones hang too heavily on the cross strings which causes then to sag down and therefore give an error. Any lighter than 8 oz and it won't keep the vertical string tight enough.
8 oz ones are available for about $10 in your local home center or from:
Having one 16 oz one can be useful for one off use down high walls.
16 oz ones are available for about $13 from:
http://www.antonline.com/p_Stanley--47-974--Stanley-47-974-16-oz-Brass-Plumb-Bob-_342898.htm